This column originally appeared in the Winter, 2002 issue of TRIUMPH.Dear Linda,
It seems to me there are several questions here. What is the knee reflex? How does polio affect it? And can the presence or absence of the knee reflex alone be used to determine whether a person has had polio?
To begin, the knee reflex you refer to is called a deep tendon reflex (DTR). DTR’s are tested to obtain information about the nerve supply to a specific muscle. Doctors can elicit a DTR by tapping gently on the tendon of a muscle with a small rubber hammer, called a reflex hammer. The tendon is the part of the muscle that attaches it to the bone. DTR’s are commonly tested in the quadriceps, biceps and gastrocnemius muscles, located in the thigh, arm and calf respectively.
Testing the knee reflex is a simple means of obtaining some information about the nerves that make your quadriceps muscle work. The quadriceps (commonly called “the quads”) is the large muscle that covers the entire front of your thigh. The quadriceps tendon attaches this muscle to the tibia, one of two bones in your lower leg. You can find your quad tendon by crossing your legs and placing your fingers just below the kneecap of your top leg. A reflex is like a communication loop. When the doctor gently taps on your quad tendon a message is sent to your spinal cord where it stimulates nerve cells, called motor neurons, to send another message back out to your quad, causing it to contract. The result is a small movement of your lower leg.
How do you think a polio-affected muscle would respond to being tapped with a reflex hammer? The answer depends on how many motor neurons survived the initial bout with polio. Remember the poliovirus damaged or killed motor neurons. Let’s track the path of the reflex. The doctor gently taps your quad tendon, sending the message to your spinal cord. If none of the motor neurons for your quad survived, then the message can go no further. The communication loop has been completely disrupted. There are no living motor neurons there to send the message back out to your quad. So, your quad would not contract; there would be no movement of your lower leg. The medical term for this is areflexia, meaning the reflex is absent. However, if some of the motor neurons survived, some information could still get through. The communication loop is only partially disrupted. These neurons could send a message out and some parts of your quad would contract. Your lower leg would move, but the movement would be smaller than if all the neurons were intact. The medical term for this is hyporeflexia, meaning weakened reflex. So a polio-affected muscle might have a weakened reflex or no reflex at all, depending on how many neurons survived.
This leads us to the last question: can the presence or absence of a reflex be used to determine whether you had polio? According to multiple sources in the Post-Polio literature, the information required to determine a polio history includes a medical history, physical examination and electromyography (EMG). EMG is considered to be the definitive test. EMG is a diagnostic test that detects the electrical activity in muscles. Polio-affected muscles have specific EMG characteristics and these findings are used to confirm a history of polio. While reflex testing is an important component of the physical exam, it would not, according to the literature, be used alone to confirm or rule out a history of polio. It is important to remember that this information refers to polio not Post-Polio Syndrome. Confirming a history of polio is only one step in the diagnosis of PPS.
I hope this has helped to clear up your confusion!